Rare earth magnets, particularly sintered R—Fe—B magnets (R is at least one of rare earth elements including Y, indispensably containing Nd), are widely used because of high magnetic properties, but Nd and Fe contained as main components are rusted very easily. To improve their corrosion resistance, the magnets are provided with corrosion-resistant coatings. Among them, a nickel electroplating is widely used for these magnets, because it has high hardness and is easier than electroless plating in controlling plating steps.
In the earliest growing stage of the electroplated nickel layer, components in an article to be plated may be dissolved in a plating solution. Particularly when the plating solution has pH on the acidic side, the article to be plated is easily dissolved and accumulated as impurities in the plating solution.
In the case of the sintered R—Fe—B magnets, rare earth elements such as Nd, and Fe, main components, are dissolved in the plating solution as impurities. As a result of continuous plating, rare earth impurities such as Nd, and Fe, main components of the magnets, are dissolved and accumulated in the plating solution. To conduct plating without impurities, a new plating solution should be used in every plating treatment. However, the preparation of a new plating solution in each plating treatment is substantially impossible because of production cost increase.
When the plating solution contains impurities, a nickel electroplating generally tends to suffer deteriorated gloss, insufficient adhesion to an article to be plated, burnt deposits, etc. For example, when a certain level or more of rare earth elements are accumulated as impurities in the plating solution, the resultant plating has low adhesion to a magnet body, thereby suffering peeling, and double plating (interfacial delamination) caused by intermittent current during plating.
Whether or not defects such as decreased adhesion and double plating occur depends on the composition of a plating solution, plating conditions, etc., and the inventor's experiment has revealed that when the amount of rare earth impurities (mainly Nd impurity) exceeds 700 ppm, these defects likely occur. Particularly, in barrel-type plating, the double plating likely occurs because large current locally flows through an article to be plated.
When nickel electroplating is carried out in an industrial mass production scale, it is impractical from the aspect of production cost to keep a nickel-electroplating solution completely free from rare earth impurities, so that it is in general not used. However, it is desirable for quality control to limit the amount of rare earth impurities as small as not exceeding 700 ppm.
Generally conducted to remove impurities such as Fe from the nickel-electroplating solution are, (1) a method of adding a nickel compound such as nickel carbonate, etc. to a plating solution to increase its pH (activated carbon may be added simultaneously to remove organic impurities), precipitating impurities by air stirring, and then removing the impurities by filtration; and (2) a method of immersing an iron net or plate in the plating solution, and conducting cathodic electrolysis at a low current density. Though these methods are effective to remove iron and organic impurities dissolved in the nickel-electroplating solution, they are not effective to remove rare earth impurities.
JP 7-62600 A discloses a method for removing rare earth impurities from a nickel-electroplating solution, with an agent used for the purification and separation of rare earth metals. This method appears to be effective as one of methods for reducing the amounts of rare earth impurities in the nickel-electroplating solution. However, this method inefficiently needs complicated steps, and impractically needs a special agent.